Nogo receptor decoy promotes recovery and corticospinal growth in non-human primate spinal cord injury

Pharmacological intervention for chronic phase of spinal cord injury

Spinal cord injury is an intractable traumatic injury. The most common hurdles faced during spinal cord injury are failure of axonal regrowth and reconnection to target sites. These also tend to be the most challenging issues in spinal cord injury. As spinal cord injury progresses to the chronic phase, lost motor and sensory functions are not recovered. Several reasons may be attributed to the failure of recovery from chronic spinal cord injury. These include factors that inhibit axonal growth such as activated astrocytes, chondroitin sulfate proteoglycan, myelin-associated proteins, inflammatory microglia, and fibroblasts that accumulate at lesion sites. Skeletal muscle atrophy due to denervation is another chronic and detrimental spinal cord injury-specific condition. Although several intervention strategies based on multiple outlooks have been attempted for treating spinal cord injury, few approaches have been successful. To treat chronic spinal cord injury, neural cells or tissue substitutes may need to be supplied in the cavity area to enable possible axonal growth. Additionally, stimulating axonal growth activity by extrinsic factors is extremely important and essential for maintaining the remaining host neurons and transplanted neurons. This review focuses on pharmacotherapeutic approaches using small compounds and proteins to enable axonal growth in chronic spinal cord injury. This review presents some of these candidates that have shown promising outcomes in basic research ( in vivo animal studies) and clinical trials: AA-NgR(310)ecto-Fc (AXER-204), fasudil, phosphatase and tensin homolog protein antagonist peptide 4, chondroitinase ABC, intracellular sigma peptide, (-)-epigallocatechin gallate, matrine, acteoside, pyrvate kinase M2, diosgenin, granulocyte-colony stimulating factor, and fampridine-sustained release. Although the current situation suggests that drug-based therapies to recover function in chronic spinal cord injury are limited, potential candidates have been identified through basic research, and these candidates may be subjects of clinical studies in the future. Moreover, cocktail therapy comprising drugs with varied underlying mechanisms may be effective in treating the refractory status of chronic spinal cord injury.

Advances in spinal cord injury: insights from non-human primates

Spinal cord injury results in significant sensorimotor deficits, currently, there is no curative treatment for the symptoms induced by spinal cord injury. Basic and pre-clinical research on spinal cord injury relies on the development and characterization of appropriate animal models. These models should replicate the symptoms observed in human, allowing for the exploration of functional deficits and investigation into various aspects of physiopathology of spinal cord injury. Non-human primates, due to their close phylogenetic association with humans, share more neuroanatomical, genetic, and physiological similarities with humans than rodents. Therefore, the responses to spinal cord injury in nonhuman primates most likely resemble the responses to traumatism in humans. In this review, we will discuss nonhuman primate models of spinal cord injury, focusing on in vivo assessments, including behavioral tests, magnetic resonance imaging, and electrical activity recordings, as well as ex vivo histological analyses. Additionally, we will present therapeutic strategies developed in non-human primates and discuss the unique specificities of non-human primate models of spinal cord injury.

Quantifying neurodegeneration of the cervical cord and brain in degenerative cervical myelopathy: A multicentre study using quantitative magnetic resonance imaging

BACKGROUND AND PURPOSE

Simultaneous assessment of neurodegeneration in both the cervical cord and brain across multiple centres can enhance the effectiveness of clinical trials. Thus, this study aims to simultaneously assess microstructural changes in the cervical cord and brain above the stenosis in degenerative cervical myelopathy (DCM) using quantitative magnetic resonance imaging (MRI) in a multicentre study.

METHODS

We applied voxelwise analysis with a probabilistic brain/spinal cord template embedded in statistical parametric mappin (SPM-BSC) to process multi parametric mapping (MPM) including effective transverse relaxation rate (R2*), longitudinal relaxation rate (R1), and magnetization transfer (MT), which are indirectly sensitive to iron and myelin content. Regression analysis was conducted to establish associations between neurodegeneration and clinical impairment. Thirty-eight DCM patients (mean age ± SD = 58.45 ± 11.47 years) and 38 healthy controls (mean age ± SD = 41.18 ± 12.75 years) were recruited at University Hospital Balgrist, Switzerland and Toronto Western Hospital, Canada.

RESULTS

Remote atrophy was observed in the cervical cord (p = 0.002) and in the left thalamus (0.026) of the DCM group. R1 was decreased in the periaqueductal grey matter (p = 0.014), thalamus (p = 0.001), corpus callosum (p = 0.0001), and cranial corticospinal tract (p = 0.03). R2* was increased in the primary somatosensory cortices (p = 0.008). Sensory impairments were associated with increased iron-sensitive R2* in the thalamus and periaqueductal grey matter in DCM.

CONCLUSIONS

Simultaneous assessment of the spinal cord and brain revealed DCM-induced demyelination, iron deposition, and atrophy. The extent of remote neurodegeneration was associated with sensory impairment, highlighting the intricate and expansive nature of microstructural neurodegeneration in DCM, reaching beyond the stenosis level.

MicroRNA-9 promotes axon regeneration of mauthner-cell in zebrafish via her6/ calcium activity pathway

MicroRNA (miRNA), functioning as a post-transcriptional regulatory element, plays a significant role in numerous regulatory mechanisms and serves as a crucial intrinsic factor influencing axon regeneration. Prior investigations have elucidated the involvement of miRNA-9 in various processes, however, its specific contribution to axon regeneration in the central nervous system (CNS) remains uncertain. Hence, the zebrafish Mauthner axon regeneration model was employed to manipulate the expression of miRNA-9 in single cells, revealing that upregulation of miRNA-9 facilitated axon regeneration. Additionally, her6, a downstream target gene of miRNA-9, was identified as a novel gene associated with axon regeneration. Suppression of her6 resulted in enhanced Mauthner axon regeneration, as evidenced by the significantly improved regenerative capacity observed in her6 knockout zebrafish. In addition, modulation of her6 expression affects intracellular calcium levels in neurons and promoting her6 expression leads to a decrease in calcium levels in vivo using the new NEMOf calcium indicator. Moreover, the administration of the neural activity activator, pentylenetetrazol (PTZ) partially compensated for the inhibitory effect of her6 overexpression on the calcium level and promoted axon regeneration. Taken together, our study revealed a role for miRNA-9 in the process of axon regeneration in the CNS, which improved intracellular calcium activity and promoted axon regeneration by inhibiting the expression of downstream target gene her6. In our study, miRNA-9 emerged as a novel and intriguing target in the intricate regulation of axon regeneration and offered compelling evidence for the intricate relationship between calcium activity and the facilitation of axon regeneration.

Lateral olfactory tract usher substance (LOTUS), an endogenous Nogo receptor antagonist, ameliorates disease progression in amyotrophic lateral sclerosis model mice

Nogo-Nogo receptor 1 (NgR1) signaling is significantly implicated in neurodegeneration in amyotrophic lateral sclerosis (ALS). We previously showed that lateral olfactory tract usher substance (LOTUS) is an endogenous antagonist of NgR1 that prevents all myelin-associated inhibitors (MAIs), including Nogo, from binding to NgR1. Here we investigated the role of LOTUS in ALS pathogenesis by analyzing G93A-mutated human superoxide dismutase 1 (SOD1) transgenic (Tg) mice, as an ALS model, as well as newly generated LOTUS-overexpressing SOD1 Tg mice. We examined expression profiles of LOTUS and MAIs and compared motor functions and survival periods in these mice. We also investigated motor neuron survival, glial proliferation in the lumbar spinal cord, and neuromuscular junction (NMJ) morphology. We analyzed downstream molecules of NgR1 signaling such as ROCK2, LIMK1, cofilin, and ataxin-2, and also neurotrophins. In addition, we investigated LOTUS protein levels in the ventral horn of ALS patients. We found significantly decreased LOTUS expression in both SOD1 Tg mice and ALS patients. LOTUS overexpression in SOD1 Tg mice increased lifespan and improved motor function, in association with prevention of motor neuron loss, reduced gliosis, increased NMJ innervation, maintenance of cofilin phosphorylation dynamics, decreased levels of ataxin-2, and increased levels of brain-derived neurotrophic factor (BDNF). Reduced LOTUS expression may enhance neurodegeneration in SOD1 Tg mice and ALS patients by activating NgR1 signaling, and in this study LOTUS overexpression significantly ameliorated ALS pathogenesis. LOTUS might serve as a promising therapeutic target for ALS.

Development of neural repair therapy for chronic spinal cord trauma: soluble Nogo receptor decoy from discovery to clinical trial

PURPOSE OF REVIEW

After traumatic spinal cord injury (SCI), neurological deficits persist due to the disconnection of surviving neurons. While repair of connectivity may restore function, no medical therapy exists today.This review traces the development of the neural repair-based therapeutic AXER-204 from animal studies to the recent clinical trial for chronic cervical SCI.

RECENT FINDINGS

Molecular studies reveal a Nogo-66 Receptor 1 (NgR1, RTN4R) pathway inhibiting axon regeneration, sprouting, and plasticity in the adult mammalian central nervous system (CNS). Rodent and nonhuman primate studies demonstrate that the soluble receptor decoy NgR(310)ecto-Fc or AXER-204 promotes neural repair and functional recovery in transection and contusion SCI. Recently, this biological agent completed a first-in-human and randomized clinical trial for chronic cervical SCI. The intervention was safe and well tolerated. Across all participants, upper extremity strength did not improve with treatment. However, posthoc and biomarker analyses suggest that AXER-204 may benefit treatment-naïve patients with incomplete SCI in the chronic stage.

SUMMARY

NgR1 signaling restricts neurological recovery in animal studies of CNS injury. The recent clinical trial of AXER-204 provides encouraging signals supporting future focused trials of this neural repair therapeutic. Further, AXER-204 studies provide a roadmap for the development of additional and synergistic therapies for chronic SCI.

Recent progress and challenges in the treatment of spinal cord injury

Spinal cord injury (SCI) disrupts the structural and functional connectivity between the higher center and the spinal cord, resulting in severe motor, sensory, and autonomic dysfunction with a variety of complications. The pathophysiology of SCI is complicated and multifaceted, and thus individual treatments acting on a specific aspect or process are inadequate to elicit neuronal regeneration and functional recovery after SCI. Combinatory strategies targeting multiple aspects of SCI pathology have achieved greater beneficial effects than individual therapy alone. Although many problems and challenges remain, the encouraging outcomes that have been achieved in preclinical models offer a promising foothold for the development of novel clinical strategies to treat SCI. In this review, we characterize the mechanisms underlying axon regeneration of adult neurons and summarize recent advances in facilitating functional recovery following SCI at both the acute and chronic stages. In addition, we analyze the current status, remaining problems, and realistic challenges towards clinical translation. Finally, we consider the future of SCI treatment and provide insights into how to narrow the translational gap that currently exists between preclinical studies and clinical practice. Going forward, clinical trials should emphasize multidisciplinary conversation and cooperation to identify optimal combinatorial approaches to maximize therapeutic benefit in humans with SCI.

Soluble Nogo-Receptor-Fc decoy (AXER-204) in patients with chronic cervical spinal cord injury in the USA: a first-in-human and randomised clinical trial

BACKGROUND

Spinal cord injury (SCI) causes neural disconnection and persistent neurological deficits, so axon sprouting and plasticity might promote recovery. Soluble Nogo-Receptor-Fc decoy (AXER-204) blocks inhibitors of axon growth and promotes recovery of motor function after SCI in animals. This first-in-human and randomised trial sought to determine primarily the safety and pharmacokinetics of AXER-204 in individuals with chronic SCI, and secondarily its effect on recovery.

METHODS

We conducted a two-part study in adults (aged 18-65 years) with chronic (>1 year) cervical traumatic SCI at six rehabilitation centres in the USA. In part 1, AXER-204 was delivered open label as single intrathecal doses of 3 mg, 30 mg, 90 mg, or 200 mg, with primary outcomes of safety and pharmacokinetics. Part 2 was a randomised, parallel, double-blind comparison of six intrathecal doses of 200 mg AXER-204 over 104 days versus placebo. Participants were randomly allocated (1:1) by investigators using a central electronic system, stratified in blocks of four by American Spinal Injury Association Impairment Scale grade and receipt of AXER-204 in part 1. All investigators and patients were masked to treatment allocation until at least day 169. The part 2 primary objectives were safety and pharmacokinetics, with a key secondary objective to assess change in International Standards for Neurological Classification of SCI (ISNCSCI) Upper Extremity Motor Score (UEMS) at day 169 for all enrolled participants. This trial is registered with ClinicalTrials.gov, NCT03989440, and is completed.

FINDINGS

We treated 24 participants in part 1 (six per dose; 18 men, six women), and 27 participants in part 2 (13 placebo, 14 AXER-204; 23 men, four women), between June 20, 2019, and June 21, 2022. There were no deaths and no discontinuations from the study due to an adverse event in part 1 and 2. In part 2, treatment-related adverse events were of similar incidence in AXER-204 and placebo groups (ten [71%] vs nine [69%]). Headache was the most common treatment-related adverse event (five [21%] in part 1, 11 [41%] in part 2). In part 1, AXER-204 reached mean maximal CSF concentration 1 day after dosing with 200 mg of 412 000 ng/mL (SD 129 000), exceeding those concentrations that were efficacious in animal studies. In part 2, mean changes from baseline to day 169 in ISNCSCI UEMS were 1·5 (SD 3·3) for AXER-204 and 0·9 (2·3) for placebo (mean difference 0·54, 95% CI -1·48 to 2·55; p=0·59).

INTERPRETATION

This study delivers the first, to our knowledge, clinical trial of a rationally designed pharmacological treatment intended to promote neural repair in chronic SCI. AXER-204 appeared safe and reached target CSF concentrations; exploratory biomarker results were consistent with target engagement and synaptic stabilisation. Post-hoc subgroup analyses suggest that future trials could investigate efficacy in patients with moderately severe SCI without prior AXER-204 exposure.

FUNDING

Wings for Life Foundation, National Institute of Neurological Disorders and Stroke, National Center for Advancing Translational Sciences, National Institute on Drug Abuse, and ReNetX Bio.

Spinal cord injury: molecular mechanisms and therapeutic interventions

Spinal cord injury (SCI) remains a severe condition with an extremely high disability rate. The challenges of SCI repair include its complex pathological mechanisms and the difficulties of neural regeneration in the central nervous system. In the past few decades, researchers have attempted to completely elucidate the pathological mechanism of SCI and identify effective strategies to promote axon regeneration and neural circuit remodeling, but the results have not been ideal. Recently, new pathological mechanisms of SCI, especially the interactions between immune and neural cell responses, have been revealed by single-cell sequencing and spatial transcriptome analysis. With the development of bioactive materials and stem cells, more attention has been focused on forming intermediate neural networks to promote neural regeneration and neural circuit reconstruction than on promoting axonal regeneration in the corticospinal tract. Furthermore, technologies to control physical parameters such as electricity, magnetism and ultrasound have been constantly innovated and applied in neural cell fate regulation. Among these advanced novel strategies and technologies, stem cell therapy, biomaterial transplantation, and electromagnetic stimulation have entered into the stage of clinical trials, and some of them have already been applied in clinical treatment. In this review, we outline the overall epidemiology and pathophysiology of SCI, expound on the latest research progress related to neural regeneration and circuit reconstruction in detail, and propose future directions for SCI repair and clinical applications.

How does Nogo receptor influence demyelination and remyelination in the context of multiple sclerosis?

Multiple sclerosis (MS) can progress with neurodegeneration as a consequence of chronic inflammatory mechanisms that drive neural cell loss and/or neuroaxonal dystrophy in the central nervous system. Immune-mediated mechanisms can accumulate myelin debris in the disease extracellular milieu during chronic-active demyelination that can limit neurorepair/plasticity and experimental evidence suggests that potentiated removal of myelin debris can promote neurorepair in models of MS. The myelin-associated inhibitory factors (MAIFs) are integral contributors to neurodegenerative processes in models of trauma and experimental MS-like disease that can be targeted to promote neurorepair. This review highlights the molecular and cellular mechanisms that drive neurodegeneration as a consequence of chronic-active inflammation and outlines plausible therapeutic approaches to antagonize the MAIFs during the evolution of neuroinflammatory lesions. Moreover, investigative lines for translation of targeted therapies against these myelin inhibitors are defined with an emphasis on the chief MAIF, Nogo-A, that may demonstrate clinical efficacy of neurorepair during progressive MS.

Drug screening in zebrafish larvae reveals inflammation-related modulators of secondary damage after spinal cord injury in mice

Prolonged inflammation after spinal cord injury is detrimental to recovery. To find pharmacological modulators of the inflammation response, we designed a rapid drug screening paradigm in larval zebrafish followed by testing of hit compounds in a mouse spinal cord injury model. Methods: We used reduced il-1β linked green fluorescent protein (GFP) reporter gene expression as a read-out for reduced inflammation in a screen of 1081 compounds in larval zebrafish. Hit drugs were tested in a moderate contusion model in mice for cytokine regulation, and improved tissue preservation and locomotor recovery. Results: Three compounds robustly reduced il-1β expression in zebrafish. Cimetidine, an over-the-counter H2 receptor antagonist, also reduced the number of pro-inflammatory neutrophils and rescued recovery after injury in a zebrafish mutant with prolonged inflammation. Cimetidine action on il-1β expression levels was abolished by somatic mutation of H2 receptor hrh2b, suggesting specific action. In mice, systemic treatment with Cimetidine led to significantly improved recovery of locomotor behavior as compared to controls, accompanied by decreased neuronal tissue loss and a shift towards a pro-regenerative profile of cytokine gene expression. Conclusion: Our screen revealed H2 receptor signaling as a promising target for future therapeutic interventions in spinal cord injury. This work highlights the usefulness of the zebrafish model for rapid screening of drug libraries to identify therapeutics to treat mammalian spinal cord injury.

Proprioceptors-enriched neuronal cultures from induced pluripotent stem cells from Friedreich ataxia patients show altered transcriptomic and proteomic profiles, abnormal neurite extension, and impaired electrophysiological properties

Friedreich ataxia is an autosomal recessive multisystem disorder with prominent neurological manifestations and cardiac involvement. The disease is caused by large GAA expansions in the first intron of the FXN gene, encoding the mitochondrial protein frataxin, resulting in downregulation of gene expression and reduced synthesis of frataxin. The selective loss of proprioceptive neurons is a hallmark of Friedreich ataxia, but the cause of the specific vulnerability of these cells is still unknown. We herein perform an in vitro characterization of human induced pluripotent stem cell-derived sensory neuronal cultures highly enriched for primary proprioceptive neurons. We employ neurons differentiated from healthy donors, Friedreich ataxia patients and Friedreich ataxia sibling isogenic control lines. The analysis of the transcriptomic and proteomic profile suggests an impairment of cytoskeleton organization at the growth cone, neurite extension and, at later stages of maturation, synaptic plasticity. Alterations in the spiking profile of tonic neurons are also observed at the electrophysiological analysis of mature neurons. Despite the reversal of the repressive epigenetic state at the FXN locus and the restoration of FXN expression, isogenic control neurons retain many features of Friedreich ataxia neurons. Our study suggests the existence of abnormalities affecting proprioceptors in Friedreich ataxia, particularly their ability to extend towards their targets and transmit proper synaptic signals. It also highlights the need for further investigations to better understand the mechanistic link between FXN silencing and proprioceptive degeneration in Friedreich ataxia.

Activation of MAP2K signaling by genetic engineering or HF-rTMS promotes corticospinal axon sprouting and functional regeneration

Facilitating axon regeneration in the injured central nervous system remains a challenging task. RAF-MAP2K signaling plays a key role in axon elongation during nervous system development. Here, we show that conditional expression of a constitutively kinase-activated BRAF in mature corticospinal neurons elicited the expression of a set of transcription factors previously implicated in the regeneration of zebrafish retinal ganglion cell axons and promoted regeneration and sprouting of corticospinal tract (CST) axons after spinal cord injury in mice. Newly sprouting axon collaterals formed synaptic connections with spinal interneurons, resulting in improved recovery of motor function. Noninvasive suprathreshold high-frequency repetitive transcranial magnetic stimulation (HF-rTMS) activated the BRAF canonical downstream effectors MAP2K1/2 and modulated the expression of a set of regeneration-related transcription factors in a pattern consistent with that induced by BRAF activation. HF-rTMS enabled CST axon regeneration and sprouting, which was abolished in MAP2K1/2 conditional null mice. These data collectively demonstrate a central role of MAP2K signaling in augmenting the growth capacity of mature corticospinal neurons and suggest that HF-rTMS might have potential for treating spinal cord injury by modulating MAP2K signaling.

MiR-33-5p alleviates spinal cord injury in rats and protects PC12 cells from lipopolysaccharide-induced apoptosis

MicroRNAs (miRNAs) exert critical effects in spinal cord injury (SCI). The miR-33-5p level is found to be lower in rats with SCI compared with that in control (untreated) and sham-operated (laminectomy but no contusion) rats. Therefore, we investigated the biological functions of miR-33-5p and related mechanisms in SCI pathogenesis and development. An in vivo SCI model and a lipopolysaccharide (LPS)-induced cell model of SCI were established. A downregulated level of miR-33-5p in experimental SCI and in LPS-treated PC12 cells was revealed by reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR). MiR-33-5p upregulation alleviated the leakage of the blood-spinal cord barrier (BSCB) induced by SCI and improved the neurological functions of SCI rats, as evidenced by the Basso, Beattie, and Bresnahan (BBB) scores and Evans blue staining. The regulatory relationship between miR-33-5p and Rps6kb1 was verified by luciferase reporter assays, which demonstrated that miR-33-5p bound to the Rps6kb1 3'UTR. Moreover, as MTT assays and flow cytometry showed, the suppressive effects of miR-33-5p upregulation on cell apoptosis were attenuated by Rps6kb1 upregulation. In conclusion, miR-33-5p ameliorates SCI in rats and inhibits the LPS-induced apoptosis of PC12 cells.

Molecular approaches for spinal cord injury treatment

Injuries to the spinal cord result in permanent disabilities that limit daily life activities. The main reasons for these poor outcomes are the limited regenerative capacity of central neurons and the inhibitory milieu that is established upon traumatic injuries. Despite decades of research, there is still no efficient treatment for spinal cord injury. Many strategies are tested in preclinical studies that focus on ameliorating the functional outcomes after spinal cord injury. Among these, molecular compounds are currently being used for neurological recovery, with promising results. These molecules target the axon collapsed growth cone, the inhibitory microenvironment, the survival of neurons and glial cells, and the re-establishment of lost connections. In this review we focused on molecules that are being used, either in preclinical or clinical studies, to treat spinal cord injuries, such as drugs, growth and neurotrophic factors, enzymes, and purines. The mechanisms of action of these molecules are discussed, considering traumatic spinal cord injury in rodents and humans.

Modulation of the Microglial Nogo-A/NgR Signaling Pathway as a Therapeutic Target for Multiple Sclerosis

Current therapeutics targeting chronic phases of multiple sclerosis (MS) are considerably limited in reversing the neural damage resulting from repeated inflammation and demyelination insults in the multi-focal lesions. This inflammation is propagated by the activation of microglia, the endogenous immune cell aiding in the central nervous system homeostasis. Activated microglia may transition into polarized phenotypes; namely, the classically activated proinflammatory phenotype (previously categorized as M1) and the alternatively activated anti-inflammatory phenotype (previously, M2). These transitional microglial phenotypes are dynamic states, existing as a continuum. Shifting microglial polarization to an anti-inflammatory status may be a potential therapeutic strategy that can be harnessed to limit neuroinflammation and further neurodegeneration in MS. Our research has observed that the obstruction of signaling by inhibitory myelin proteins such as myelin-associated inhibitory factor, Nogo-A, with its receptor (NgR), can regulate microglial cell function and activity in pre-clinical animal studies. Our review explores the microglial role and polarization in MS pathology. Additionally, the potential therapeutics of targeting Nogo-A/NgR cellular mechanisms on microglia migration, polarization and phagocytosis for neurorepair in MS and other demyelination diseases will be discussed.

Targeting RTN4/NoGo-Receptor reduces levels of ALS protein ataxin-2

Gene-based therapeutic strategies to lower ataxin-2 levels are emerging for the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and spinocerebellar ataxia type 2 (SCA2). Additional strategies to lower levels of ataxin-2 could be beneficial. Here, we perform a genome-wide arrayed small interfering RNA (siRNA) screen in human cells and identify RTN4R, the gene encoding the RTN4/NoGo-Receptor, as a potent modifier of ataxin-2 levels. RTN4R knockdown, or treatment with a peptide inhibitor, is sufficient to lower ataxin-2 protein levels in mouse and human neurons in vitro, and Rtn4r knockout mice have reduced ataxin-2 levels in vivo. We provide evidence that ataxin-2 shares a role with the RTN4/NoGo-Receptor in limiting axonal regeneration. Reduction of either protein increases axonal regrowth following axotomy. These data define the RTN4/NoGo-Receptor as a novel therapeutic target for ALS and SCA2 and implicate the targeting of ataxin-2 as a potential treatment following nerve injury.

Rabphilin3A reduces integrin-dependent growth cone signaling to restrict axon regeneration after trauma

Neural repair after traumatic spinal cord injury depends upon the restoration of neural networks via axonal sprouting and regeneration. Our previous genome wide loss-of-function screen identified Rab GTPases as playing a prominent role in preventing successful axon sprouting and regeneration. Here, we searched for Rab27b interactors and identified Rabphilin3A as an effector within regenerating axons. Growth cone Rabphilin3a colocalized and physically associated with integrins at puncta in the proximal body of the axonal growth cone. In regenerating axons, loss of Rabphilin3a increased integrin enrichment in the growth cone periphery, enhanced focal adhesion kinase activation, increased F-actin-rich filopodial density and stimulated axon extension. Compared to wild type, mice lacking Rabphilin3a exhibited greater regeneration of retinal ganglion cell axons after optic nerve crush as well as greater corticospinal axon regeneration after complete thoracic spinal cord crush injury. After moderate spinal cord contusion injury, there was greater corticospinal regrowth in the absence of Rph3a. Thus, an endogenous Rab27b - Raphilin3a pathway limits integrin action in the growth cone, and deletion of this monomeric GTPase pathway permits reparative axon growth in the injured adult mammalian central nervous system.

Proteoglycans and Glycosaminoglycans in Central Nervous System Injury

The brain and spinal cord constitute the central nervous system (CNS), which when injured, can be exceedingly devastating. The mechanistic roles of proteoglycans (PGs) and their glycosaminoglycan (GAG) side chains in such injuries have been extensively studied. CNS injury immediately alters endothelial and extracellular matrix (ECM) PGs and GAGs. Subsequently, these alterations contribute to acute injury, post-injury fibrosis, and post-injury repair. These effects are central to the pathophysiology of CNS injury. This review focuses on the importance of PGs and GAGs in multiple forms of injury including traumatic brain injury, spinal cord injury, and stroke. We highlight the causes and consequences of degradation of the PG and GAG-enriched endothelial glycocalyx in early injury and discuss the pleiotropic roles of PGs in neuroinflammation. We subsequently evaluate the dualistic effects of PGs on recovery: both PG/GAG-mediated inhibition and facilitation of repair. We then report promising therapeutic strategies that may prove effective for repair of CNS injury including PG receptor inhibition, delivery of endogenous, pro-repair PGs and GAGs, and direct degradation of pathologic GAGs. Last, we discuss importance of two PG- and GAG-containing ECM structures (synapses and perineuronal nets) in CNS injury and recovery.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

The Role of Tissue Geometry in Spinal Cord Regeneration

Unlike peripheral nerves, axonal regeneration is limited following injury to the spinal cord. While there may be reduced regenerative potential of injured neurons, the central nervous system (CNS) white matter environment appears to be more significant in limiting regrowth. Several factors may inhibit regeneration, and their neutralization can modestly enhance regrowth. However, most investigations have not considered the cytoarchitecture of spinal cord white matter. Several lines of investigation demonstrate that axonal regeneration is enhanced by maintaining, repairing, or reconstituting the parallel geometry of the spinal cord white matter. In this review, we focus on environmental factors that have been implicated as putative inhibitors of axonal regeneration and the evidence that their organization may be an important determinant in whether they inhibit or promote regeneration. Consideration of tissue geometry may be important for developing successful strategies to promote spinal cord regeneration.

Inositol Polyphosphate-5-Phosphatase K (Inpp5k) Enhances Sprouting of Corticospinal Tract Axons after CNS Trauma

Failure of CNS neurons to mount a significant growth response after trauma contributes to chronic functional deficits after spinal cord injury. Activator and repressor screening of embryonic cortical neurons and retinal ganglion cells in vitro and transcriptional profiling of developing CNS neurons harvested in vivo have identified several candidates that stimulate robust axon growth in vitro and in vivo Building on these studies, we sought to identify novel axon growth activators induced in the complex adult CNS environment in vivo We transcriptionally profiled intact sprouting adult corticospinal neurons (CSNs) after contralateral pyramidotomy (PyX) in nogo receptor-1 knock-out mice and found that intact CSNs were enriched in genes in the 3-phosphoinositide degradation pathway, including six 5-phosphatases. We explored whether inositol polyphosphate-5-phosphatase K (Inpp5k) could enhance corticospinal tract (CST) axon growth in preclinical models of acute and chronic CNS trauma. Overexpression of Inpp5k in intact adult CSNs in male and female mice enhanced the sprouting of intact CST terminals after PyX and cortical stroke and sprouting of CST axons after acute and chronic severe thoracic spinal contusion. We show that Inpp5k stimulates axon growth in part by elevating the density of active cofilin in labile growth cones, thus stimulating actin polymerization and enhancing microtubule protrusion into distal filopodia. We identify Inpp5k as a novel CST growth activator capable of driving compensatory axon growth in multiple complex CNS injury environments and underscores the veracity of using in vivo transcriptional screening to identify the next generation of cell-autonomous factors capable of repairing the damaged CNS.SIGNIFICANCE STATEMENT Neurologic recovery is limited after spinal cord injury as CNS neurons are incapable of self-repair post-trauma. In vitro screening strategies exploit the intrinsically high growth capacity of embryonic CNS neurons to identify novel axon growth activators. While promising candidates have been shown to stimulate axon growth in vivo, concomitant functional recovery remains incomplete. We identified Inpp5k as a novel axon growth activator using transcriptional profiling of intact adult corticospinal tract (CST) neurons that had initiated a growth response after pyramidotomy in plasticity sensitized nogo receptor-1-null mice. Here, we show that Inpp5k overexpression can stimulate CST axon growth after pyramidotomy, stroke, and acute and chronic contusion injuries. These data support in vivo screening approaches to identify novel axon growth activators.

Neural Stem Cells Overexpressing Nerve Growth Factor Improve Functional Recovery in Rats Following Spinal Cord Injury via Modulating Microenvironment and Enhancing Endogenous Neurogenesis

Spinal cord injury (SCI) is a devastating event characterized by severe motor, sensory, and autonomic dysfunction. Currently, there is no effective treatment. Previous studies showed neural growth factor (NGF) administration was a potential treatment for SCI. However, its targeted delivery is still challenging. In this study, neural stem cells (NSCs) were genetically modified to overexpress NGF, and we evaluated its therapeutic value following SCI. Four weeks after transplantation, we observed that NGF-NSCs significantly enhanced the motor function of hindlimbs after SCI and alleviated histopathological damage at the lesion epicenter. Notably, the survival NGF-NSCs at lesion core maintained high levels of NGF. Further immunochemical assays demonstrated the graft of NGF-NSCs modulated the microenvironment around lesion core via reduction of oligodendrocyte loss, attenuation of astrocytosis and demyelination, preservation of neurons, and increasing expression of multiple growth factors. More importantly, NGF-NSCs seemed to crosstalk with and activate resident NSCs, and high levels of NGF activated TrkA, upregulated cAMP-response element binding protein (CREB) and microRNA-132 around the lesion center. Taken together, the transplantation of NGF-NSCs in the subacute stage of traumatic SCI can facilitate functional recovery by modulating the microenvironment and enhancing endogenous neurogenesis in rats. And its neuroprotective effect may be mediated by activating TrkA, up-regulation of CREB, and microRNA-132.

Spinal Cord Injury: A Review of Current Management Considerations and Emerging Treatments

Traumatic spinal cord injuries can have devastating outcomes for patients. In this focused review, we discuss the epidemiology of spinal cord injuries, associated neurologic exam findings, and primary and secondary injury progression. We then delve into the emerging treatment approaches and relevance to improving outcomes. The disease is multifactorial and has many management considerations. This concise user-friendly resource can help guide clinicians caring for these patients. Also, it points to the need for continued scientific discovery and improved pharmaceutical and device innovations.

When Spinal Neuromodulation Meets Sensorimotor Rehabilitation: Lessons Learned From Animal Models to Regain Manual Dexterity After a Spinal Cord Injury

Electrical neuromodulation has strongly hit the foundations of spinal cord injury and repair. Clinical and experimental studies have demonstrated the ability to neuromodulate and engage spinal cord circuits to recover volitional motor functions lost after the injury. Although the science and technology behind electrical neuromodulation has attracted much of the attention, it cannot be obviated that electrical stimulation must be applied concomitantly to sensorimotor rehabilitation, and one would be very difficult to understand without the other, as both need to be finely tuned to efficiently execute movements. The present review explores the difficulties faced by experimental and clinical neuroscientists when attempting to neuromodulate and rehabilitate manual dexterity in spinal cord injured subjects. From a translational point of view, we will describe the major rehabilitation interventions employed in animal research to promote recovery of forelimb motor function. On the other hand, we will outline some of the state-of-the-art findings when applying electrical neuromodulation to the spinal cord in animal models and human patients, highlighting how evidences from lumbar stimulation are paving the path to cervical neuromodulation.

NogoA-expressing astrocytes limit peripheral macrophage infiltration after ischemic brain injury in primates

Astrocytes play critical roles after brain injury, but their precise function is poorly defined. Utilizing single-nuclei transcriptomics to characterize astrocytes after ischemic stroke in the visual cortex of the marmoset monkey, we observed nearly complete segregation between stroke and control astrocyte clusters. Screening for the top 30 differentially expressed genes that might limit stroke recovery, we discovered that a majority of astrocytes expressed RTN4A/ NogoA, a neurite-outgrowth inhibitory protein previously only associated with oligodendrocytes. NogoA upregulation on reactive astrocytes post-stroke was significant in both the marmoset and human brain, whereas only a marginal change was observed in mice. We determined that NogoA mediated an anti-inflammatory response which likely contributes to limiting the infiltration of peripheral macrophages into the surviving parenchyma.

Restoring Sensorimotor Function Through Neuromodulation After Spinal Cord Injury: Progress and Remaining Challenges

Spinal cord injury (SCI) is a major disability that results in motor and sensory impairment and extensive complications for the affected individuals which not only affect the quality of life of the patients but also result in a heavy burden for their families and the health care system. Although there are few clinically effective treatments for SCI, research over the past few decades has resulted in several novel treatment strategies which are related to neuromodulation. Neuromodulation-the use of neuromodulators, electrical stimulation or optogenetics to modulate neuronal activity-can substantially promote the recovery of sensorimotor function after SCI. Recent studies have shown that neuromodulation, in combination with other technologies, can allow paralyzed patients to carry out intentional, controlled movement, and promote sensory recovery. Although such treatments hold promise for completely overcoming SCI, the mechanisms by which neuromodulation has this effect have been difficult to determine. Here we review recent progress relative to electrical neuromodulation and optogenetics neuromodulation. We also examine potential mechanisms by which these methods may restore sensorimotor function. We then highlight the strengths of these approaches and remaining challenges with respect to its application.

Regenerated interneurons integrate into locomotor circuitry following spinal cord injury

Whereas humans and other adult mammals lack the ability to regain locomotor function after spinal cord injury, zebrafish are able to recover swimming behavior even after complete spinal cord transection. We have previously shown that zebrafish larvae regenerate lost spinal cord neurons within 9 days post-injury (dpi), but it is unknown whether these neurons are physiologically active or integrate into functional circuitry. Here we show that genetically defined premotor interneurons are regenerated in injured spinal cord segments as functional recovery begins. Further, we show that these newly-generated interneurons receive excitatory input and fire synchronously with motor output by 9 dpi. Taken together, our data indicate that regenerative neurogenesis in the zebrafish spinal cord produces interneurons with the ability to integrate into existing locomotor circuitry.

Pharmacological management of secondary spinal cord injury

Introduction: Secondary spinal cord injury (SCI) sets on immediately after trauma and, despite prompt treatment, may become chronic. SCI is a complex condition and presents numerous challenges to patients and physicians alike, also considering the lack of an approved pharmacological therapy.Areas covered: This review describes the pathophysiological mechanisms leading to secondary SCI to highlight possible targets for pharmacological therapy. Furthermore, an extensive search of the literature on different databases (PubMed, Google scholar, Embase, and Scopus) and of the current clinical trials (clinicaltrials.gov) was performed to investigate the current outlook for the pharmacological management of SCI. Only drugs with performed or ongoing clinical trials were considered.Expert opinion: Pharmacological therapy aims to improve motor and sensory function in patients. Overall, drugs are divided into neuroprotective compounds, which aim to limit the damage induced by the pro-inflammatory and pro-apoptotic milieu of SCI, and neuroregenerative drugs, which induce neuronal and axonal regrowth. While many compounds have been trialed with promising results, none has yet completed a stage III trial and has been approved for the pharmacological management of SCI.

Nanopharmaceutical-based regenerative medicine: a promising therapeutic strategy for spinal cord injury

Spinal cord injury (SCI) is a neurological disorder that can lead to loss of perceptive and athletic function due to the severe nerve damage. To date, pieces of evidence detailing the precise pathological mechanisms in SCI are still unclear. Therefore, drug therapy cannot effectively alleviate the SCI symptoms and faces the limitations of systemic administration with large side effects. Thus, the development of SCI treatment strategies is urgent and valuable. Due to the application of nanotechnology in pharmaceutical research, nanopharmaceutical-based regenerative medicine will bring colossal development space for clinical medicine. These nanopharmaceuticals (i.e. nanocrystalline drugs and nanocarrier drugs) are designed using different types of materials or bioactive molecules, so as to improve the therapeutic effects, reduce side effects, and subtly deliver drugs, etc. Currently, an increasing number of nanopharmaceutical products have been approved by drug regulatory agencies, which has also prompted more researchers to focus on the potential treatment strategies of SCI. Therefore, the purpose of this review is to summarize and elaborate the research progress as well as the challenges and future of nanopharmaceuticals in the treatment of SCI, aiming to promote further research of nanopharmaceuticals in SCI.

Optic nerve regeneration screen identifies multiple genes restricting adult neural repair

Adult mammalian central nervous system (CNS) trauma interrupts neural networks and, because axonal regeneration is minimal, neurological deficits persist. Repair via axonal growth is limited by extracellular inhibitors and cell-autonomous factors. Based on results from a screen in vitro, we evaluate nearly 400 genes through a large-scale in vivo regeneration screen. Suppression of 40 genes using viral-driven short hairpin RNAs (shRNAs) promotes retinal ganglion cell (RGC) axon regeneration after optic nerve crush (ONC), and most are validated by separate CRISPR-Cas9 editing experiments. Expression of these axon-regeneration-suppressing genes is not significantly altered by axotomy. Among regeneration-limiting genes, loss of the interleukin 22 (IL-22) cytokine allows an early, yet transient, inflammatory response in the retina after injury. Reduced IL-22 drives concurrent activation of signal transducer and activator of transcription 3 (Stat3) and dual leucine zipper kinase (DLK) pathways and upregulation of multiple neuron-intrinsic regeneration-associated genes (RAGs). Including IL-22, our screen identifies dozens of genes that limit CNS regeneration. Suppression of these genes in the context of axonal damage could support improved neural repair.

Hydrogels as delivery systems for spinal cord injury regeneration

Spinal cord injury is extremely debilitating, both at physiological and psychological levels, changing completely the patient's lifestyle. The introduction of biomaterials has opened a new window to develop a therapeutic approach to induce regeneration after injury due to similarities with extracellular matrix. Particularly, hydrogels have the ability to support axonal growth and endogenous regeneration. Moreover, they can also act as potential matrixes in which to load and deliver therapeutic agents at injury site. In this review, we highlight some important characteristics to be considered when designing hydrogels as delivery systems (DS), such as rheology, mesh size, swelling, degradation, gelation temperature and surface charge. Additionally, affinity-based release systems, incorporation of nanoparticles, or ion-mediated interactions are also pondered. Overall, hydrogel DS aim to promote a sustained, controlled and prolonged release at injury site, allowing a targeted oriented action of the therapeutic agent that will be used.

Inhibiting an inhibitor: a decoy to recover dexterity after spinal cord injury

This scientific commentary refers to ‘Nogo receptor decoy promotes recovery and corticospinal growth in non-human primate spinal cord injury’, by Wang et al. (doi:10.1093/brain/awaa116).